ولو كان مسافراً في أول الوقت مقيماً في آخره يصلي أربعاً
A person who is a traveller at the start but a resident at the end performs four units.
Suppose the period for a four-unit prayer opens during travel, but the person reaches home without praying and is a resident in the relevant final portion. The obligation is performed in the resident form.
It would be mistaken to infer the shorter form solely from the person's location at the opening of the time. The rule concerns the status governing an obligation that has remained unperformed.